A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example to transport water throughout the RPV. For example, core spray piping is routed over and through the shroud to deliver water to the reactor core.
Intergranular Stress Corrosion Cracking (IGSCC) is a known phenomenon occurring in reactor components, such as structural members, shrouds, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to IGSCC.
Over the life of the reactor, the shroud, shroud support, baffle plate, shroud support legs and the attachment welds situated in the reactor are susceptible to various types of cracking due to the hostile nature of the environment in the reactor. It is important to examine these components periodically to determine whether any cracking has occurred. Furthermore, in cases where major repair and refurbishment is undertaken in the reactor, such as shroud replacement, it is important to know if the components that are not being replaced, such as components that support the shroud, are free of cracks.
While examinations in the annulus, or region between the shroud and the pressure vessel wall, can be performed, these examinations are likely to be only partial inspections due to access limitations in the annular region of the reactor. Any examination in the annulus is hampered by the numerous components such as jet pumps and further constrained by any shroud repair hardware present in the annulus. Thus, it is highly advantageous to perform the examination of the components from the inside of the shroud cylinder.
It would be desirable to provide an inspection tool that is capable of performing ultrasonic examinations of shrouds and shroud components from within the shroud cylinder in a nuclear reactor. It would also be desirable to provide an inspection tool that is remotely operable and capable of providing position information relative to fixed items within the reactor.